Previously we combined commonly used Protamine Sulfate (Pro) with superparamagnetic dextran coated iron oxide nanoparticle (SPIO) Ferumoxides (FE), a complex is formed that can be used to magnetically label stem cells and other mammalian cells. Cells take up FEPro complexes by macropinocytosis. Early detection of labeled cells in vivo by cellular MRI requires the development of novel pulse sequences or image processing to improve the sensitivity to low numbers of iron oxide labeled cells in tissues. However, ferumoxides were taken off the market place and ferumoxytol (F) an FDA approved iron oxide nanoparticle used to treat iron deficiency anemia for chronic kidney disease was introduced in 2009. Ferumoxytol (F) is a nanoparticle of 17-31 nm in diameter with the 6 nm iron oxide crystals core and has been also used in experimental and clinical trials as a macrophage imaging agent as well as blood pool agent with MRI. We demonstrated that by combining F with Pro and heparin (H) resulted in a self assembling nanocomplex that could be used to label cells. Transmission electron microscopy of HPF revealed that these nanocomplexes were spheroid shaped with the HP in center surrounded by F. Incubating stem cells and T-cells in HFP nanocomplexes in serum free media for 2 hours followed by complete media resulted in cell labeling. HPF labeling did not impair the cells viability, proliferative capacity, apoptotic rate, activation, phenotypic surface marker expression, or capacity to differentiate. MRI at 3T of HPF labeled BMSC implanted in the rat brain demonstrated the ability to detect 1000 cells with a 50% decrease in T2* in the rat brain compared to the surrounding parenchyma. Pre-clinical safety/toxicity studies of intracerebrally administrated HPF-labeled NSCs in mice were also performed, and demonstrated no significant clinical or behavioral changes, no neuronal or systemic toxicities, and no abnormal accumulation of iron in the liver or spleen. The HPF labeling technique has been scaled up, and the NIH Cell Processing Section cGMP facility was able to label BMSCs in biofactories with no changes in BMSC function or viability of the labeled cryopreserved cell product. The HPF method was also used to label genetically engineered neural stem cells (NSC) that express cytosine deaminase as part of an ongoing clinical trial to treat patients with recurrent glioblastoma. The HPF labeled NSC are to be directly implanted deep into brain around the periphery of the surgical resection sight in order that they can migrate to areas of residual tumors or satellite metastasis. MRI monitored the migration of the HPF labeled NSC in the brain over 30 days. Initial studies performed at City of Hope report that there was essential no adverse events following the implantation of labeled cells. The HPF labeling method should facilitate the monitoring by MRI of transplanted cells in other clinical trials.